On Sat, 14 Mar 2009, Dietmar Maurer wrote:
> > Let's try to clear up the confusion.
> >
> > Kernel device-mapper (which lvm uses) does not support write barriers
> > except in very restricted circumstances (when only one device is
> > involved and the mapping is trivial). If dm receives a write barrier
> > which is not supported it notifies the caller (typically a filesystem)
> > so appropriate action can be taken if it wishes.
>
> Does that mean I should never use more than one device if I have
> applications depending on fsync (databases)?
It just means that write barriers won't get passed to the device.
This is only a problem if the devices have write caches. Note
that with multiple devices, even a FIFO write cache could cause
reordering between devices (one device could finish faster than another).
--
Stuart D. Gathman <stuart@bmsi.com>
Business Management Systems Inc. Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flammis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.
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